Ferrofluid is typically employed in hydrodynamic bearings as a bearing fluid and often in combination of ferrofluid bearing-seal arrangements. One ferrofluid-film-bearing embodiment used commercially in computer-disc-drive applications is where the ferrofluid is employed both as a fluid bearing and as a seal, in connection with a ferrofluid seal apparatus at each end of the bearing. The main components of a ferrofluid-film-bearing device employed in the computer-disc drives comprises a radial bearing, a pair of thrust bearings and usually two ferrofluid seal apparatuses at one or each end of the bearing cavity, to retain the ferrofluid within the bearing cavity (see, for example, U.S. Pat. No. 4,526,484, issued July 2, 1985).
In computer-disc-drive applications, the speed of the bearing is or approaches 3600 rpms, which may be accomplished with the use of an in-line motor. The radial clearance in the bearing varies, but generally may be up to 0.6 mil, and the bearing may be a three- or four-sector bearing assembly.
The advantages of such a bearing assembly, over conventional ball-bearing-spindle assemblies, are extremely low nonrepetitive runout, damping and quietness of operation. The ferrofluid-film-bearing spindle in the bearing assembly, however, needs to be grounded in computer-disc-drive and other applications, since the rotating disc-drive shaft accumulates electrical charges, and, consequently, a potential difference is produced between the head of the computer disc drive and the disc. This accumulated charge on the rotating disc shaft must be grounded, in order to avoid a spark which may damage the disc or the magnetic head, with the consequent loss of information on the disc. Generally, a grounding scheme is used, wherein a grounding button is employed in contact with the shaft, to conduct built-up static charges to ground. The grounding button, however, is not fully satisfactory, since it tends to generate particles in use, produces noise, wears with time, adds a new component into the bearing and seal device, and makes the resulting bearing seal assembly more expensive and unreliable with time.
Further, in the operation of a ferrofluid bearing, the ferrofluid composition employed needs to be a low-viscosity ferrofluid composition, such as a composition having a viscosity of less than 50 cps to 27.degree. C. or more typically 25 cp or 15 cp or less. Low-viscosity ferrofluid compositions suitable for use would be an ester-based ferrofluid having a 200-gauss magnetization and viscosity of 15 cps at 27.degree. C. A low-viscosity ferrofluid provides for a low power consumption for the bearing assembly, and a small temperature rise, minimizing the thermal expansion of the components in the bearing assembly, thereby maximizing bearing performance by such thermal expansion. A small temperature rise is further necessary in reducing the viscosity changes and, thus, not affecting the load-bearing characteristics and capabilities of the bearing assembly.
Therefore, it is desirable to provide for an electrically conductive bearing apparatus and for a low-viscosity electrically conductive ferrofluid composition for use in such bearing apparatus.